Can unit frames show wrong health?

Let's say i'm doing PvP, can for example another addon UI be wrong compared to blizzard ui? that my health on the addon is 30k, but its actually 10k? this can be a disadvantage in PvP. But can it be like this?

Any addon can potentially be wrong if the way it is querying or interpreting is incorrect.
There is no special circumstance where an addon will be wrong, just one of several likely causes.
A bug in the addon itself, interference from another addon, or a bug in the game.

Is this a specific addon or problem you are asking about, or just in general ?
Seems a strange and very specific question should it simply be rhetorical.

Very much so, and I've seen it before. This is especially possible when blizzard releases a patch but a UI addon hasn't been updated yet. I've seen it happen with Xperl before, though back during Wrath or so.

Originally Posted by Warwithin

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Originally Posted by Adam Jensen

Except maybe Morgan Freeman. That man could convince God to be an atheist with that voice of his . . .

I'm using Shadowed Unit Frames, and I see most top arena players use blizzard ui. ofcourse its probably preference, but can it be that even with it updated, it can show wrong?

Addons can clash and interfere with each other or the default UI so it is entirely possible.
You mention a specific addon.
IS this question because you are seeing incorrect numbers somewhere, or simply for something else.
If we are really to try and help we need to know if you are seeing a problem or not.

One thing to keep in mind is that the default UI is built using the same mechanisms than those used by addons. So addons aren't really "second-class" and a good developer can write UI code performing just as well as the default UI.

Maybe similar in performance, but there is some functionality limited to the default UI.
That is most noticeable when it comes to in-combat restrictions, particularly on raid frames.

Theres nothing the default raid frames can do that you couldnt do with a custom addon. Yes there are some in-combat restriction, but youre using blizzard templates for things like unitframes and actionbars...

The only thing that you really cant do with an addon is custom talent frame.